Pan-based carbon fiber and production method therefor

ABSTRACT

A polyacrylonitrile (PAN)-based carbon fiber includes three or more phases different in crystal size.

TECHNICAL FIELD

This disclosure relates to a polyacrylonitrile (hereinafter, referred toas PAN)-based carbon fiber comprising three or more phases different incrystal size, and a production method therefor.

BACKGROUND

Carbon fiber is broadly used in various uses, for example, for aerospacematerials for airplanes, rockets and the like, and for sport articlessuch as tennis rackets and golf shafts and, further, is also used fortransportation and mechanical fields such as ships and vehicles, fromits properties such as mechanical and chemical properties and lightnessin weight. Further, in recent years, from high conductivity or highradiation property of carbon fiber, application to uses for parts forelectronic equipment such as housings of portable telephones or personalcomputers, or for electrodes of fuel cells, is strongly required. Inparticular, PAN-based carbon fiber, because of its high specificstrength, is used in particular for aerospace materials for airplanes,space satellites and the like, for members for vehicles and the likeand, recently, application to vehicle members is being remarkablyincreased. Therefore, it is desired to improve the productivity ofcarbon fiber.

The PAN-based carbon fiber can be obtained by inducing a polymersolution dissolved with mainly PAN into a solvent to a PAN-based fiberby spinning, and burning it at a high temperature under a condition ofan inert atmosphere. When the PAN-based fiber is made into a carbonfiber, the PAN-based fiber is passed through a process of airstabilization (cyclization reaction and oxidation reaction of PAN) whichheats the PAN-based fiber in air at a high temperature such as 200 to300° C. It is general to obtain a carbon fiber by further treating it ina carbonization furnace at 2,000° C. to 3,000° C. for several minutes.However, because an exothermic reaction progresses in the stabilizationprocess, heat removal is required when a large amount of PAN-basedfibers are stabilized. Therefore, for temperature control, a long-timetreatment is required, and it is necessary to restrict the fineness ofthe PAN-based precursor fiber to a small fineness of a specified valueor less to finish air stabilization in a desired period of time. Thus,in the known process of producing a carbon fiber, the knownstabilization process is a rate-limiting factor, and it cannot be saidto be a sufficiently efficient process.

Further, although a carbon fiber is excellent in specific strength andspecific elastic modulus, it has a defect of a very low degree ofelongation. An increase of degree of elongation of carbon fiber isstrongly desired accompanying with an increase of demand for carbonfiber. So far, to increase the degree of elongation of carbon fiber,although a fiber spun with a raw material composition, the maincomponent of which is a polymer compound compounded with an aromaticsulfonic group or a salt thereof via a methylene-type bond, is disclosed(JP 6-173122 A), there is a defect in that the cost of the main rawmaterial is too high. Further, although technologies intended to improveproperties of carbon fiber by making a hollow carbon fiber or adual-structured carbon fiber are also known (JP 2008-169511 A, JP2007-291557 A and JP 2001-73230 A), the degree of elongation thereof isstill insufficient. Therefore, a long fiber of carbon fiber having asufficient degree of elongation relative to its strength has not beenobtained.

Namely, it is required to greatly shorten the time for stabilization ofa fiber and obtain a carbon fiber having a high degree of elongation.

Accordingly, to satisfy the above-described requirements, it could behelpful to provide a PAN-based carbon fiber capable of greatlyshortening the time for stabilization of a fiber and exhibiting a highdegree of elongation while maintaining a sufficiently high strength, anda production method therefor.

SUMMARY

We thus provide a PAN-based carbon fiber comprising three or more phasesdifferent in crystal size.

In the above-described PAN-based carbon fiber, it is preferred that therespective phases are layered.

Further, it is preferred that this PAN-based carbon fiber has asheath-core structure having three or more layers, and satisfiesconditions A to D:

A: in a sectional area in a direction perpendicular to a fiber axis, anarea occupied by a core occupies 10 to 70% of the whole of the sectionalarea,B: a thickness of a sheath is in a range of 100 nm to 10,000 nm,C: a thickness of an intermediate layer is more than 0 nm and 5,000 nmor less, andD: a diameter in the direction perpendicular to the fiber axis is 2 μmor more.

Further, it is preferred that the above-described PAN-based carbon fiberhas a sheath-core structure having three or more layers, and satisfiesconditions E to H:

wherein a crystal size of a core is referred to as Lc1, a crystal sizeof a sheath is referred to as Lc2, and a crystal size of an intermediatelayer is referred to as Lc3.

E: Lc1/Lc3≧1.05, F: Lc1/Lc2≧1.05 G: 1.0≦Lc1≦7.0 nm, and H: Lc2≠Lc3

Further, in the above-described PAN-based carbon fiber having asheath-core structure with three or more layers, it is preferred that anorientation degree f of a crystal of a core is 0.7 or less.

Further, it is preferred that the PAN-based carbon fiber is obtained bycarbonizing a fiber spun from a single kind of polymer solution forspinning.

Further, it is preferred that the PAN-based carbon fiber is obtained byspinning a fiber from a polymer solution for spinning satisfying pointsA and B and carbonizing the spun fiber:

A: a polymer in the polymer solution for spinning is a polymer preparedby modifying PAN with an amine-based compound and oxidizing it with anitro compound, andB: the nitro compound is not contained in the polymer solution forspinning.

Further, in such a PAN-based carbon fiber, in the above-described Arelating to a polymer in the polymer solution for spinning, it ispreferred that it is obtained using a polymer solution for spinningcontaining PAN oxidized using a nitro compound, in particular,nitrobenzene, at an amount of 10 wt % or more relative to PAN.

Furthermore, it is preferred that the PAN-based carbon fiber is obtainedusing a polymer solution for spinning having a divergent structure inwhich a gradient a is 0.1 or more and 0.3 or less as the resultdetermined by GPC (Gel Permeation Chromatography).

wherein the gradient a means a gradient a represented by MarkHouwink-Sakurada equation (1):

[η]=KMw^(a)  (1)

wherein [η] is an intrinsic viscosity, K is a constant inherent for amaterial, and Mw is a weight average molecular weight.

A method of producing a PAN-based carbon fiber comprises the steps of:spinning the above-described polymer solution for spinning; performingstabilization in air at 280° C. or higher and 400° C. or lower for 10seconds or more and 15 minutes or less; and thereafter, performingcarbonization. In this method, it is preferred that the stabilization isperformed using an infrared heater (for example, a ceramic heater) and ahot air drier (for example, a hot air circulation drier) together.

In the PAN-based carbon fiber and the production method therefor, byconfiguring the carbon fiber from three or more phases different incrystal size, or by the production method wherein a specified polymerfor spinning is spun, stabilization is performed under specifiedconditions and thereafter carbonization is performed, the time forstabilization can be greatly shortened and productivity can be improved,and a PAN-based carbon fiber capable of exhibiting a high degree ofelongation while maintaining a sufficiently high strength can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic sectional view in a direction perpendicular toa fiber axis showing an example of a sheath-core structure having threelayers, and a partially enlarged view thereof.

FIG. 2 shows diagrams exemplifying electron diffractions of TEM(Transmission Electron Microscope) in a core, an intermediate layer anda sheath of a sheath-core structure.

FIG. 3 is a characteristic diagram showing distribution curves convertedfrom the light and shade of the electron diffraction diagrams depictedin FIG. 2.

FIG. 4 is a schematic vertical sectional view showing an example of ahot air circulation furnace equipped with an infrared heater which isused for stabilization.

EXPLANATION OF SYMBOLS

-   -   1: sheath-core structure having three or more layers    -   2: core    -   3: intermediate layer    -   4: sheath    -   11: hot air circulation drier    -   12: non-treated fiber (fiber before treatment)    -   13: stabilized fiber (fiber after treatment)    -   14 a, 14 b: roller    -   15 a, 15 b: opening    -   16: ceramic heater    -   17: punching metal for attaching ceramic heater    -   18: flow of hot air

DETAILED DESCRIPTION

Hereinafter, examples will be explained in detail.

A carbon fiber means a fiber composed of 90% or more with C (carbon)component. It is possible to determine the content of C component byelemental analysis.

It is necessary that the PAN-based carbon fiber comprises three or morephases different in crystal size. By forming three or more phases, highfunctions can be provided to the carbon fiber. Further, the carbon fiberis preferably a carbon fiber in which the above-described respectivephases are layered. By the layered structure, it tends that the strengthof the carbon fiber is maintained and the carbon fiber has a high degreeof elongation.

Further, the carbon fiber preferably forms a sheath-core structurehaving three or more layers to exhibit the desired properties. Forexample, as shown in FIG. 1, the sheath-core structure 1 having three ormore layers is a structure having an intermediate layer 3 (for example,a plurality of intermediate layers) between a core 2 and a sheath 4,which is a structure formed in three or more layers as a whole, and inparticular, it is preferred to be a structure of three layers. In thesheath-core structure having three or more layers, it is more preferredthat a crystal size of the core Lc1, a crystal size of the sheath Lc2,and a crystal size of the intermediate layer Lc3 have relationships ofLc1/Lc3≧1.05, Lc1/Lc2≧1.05, and 1.5≧Lc1≦7.0 nm. More preferably, therelationships are Lc1/Lc3≧1.10 and Lc1/Lc2≧1.08. Further preferably, therelationships are Lc1/Lc3≧1.15 and Lc1/Lc2≧1.1. Lc referred hereindicates an overlap thickness of graphite moment in a direction offiber axis. The crystal size Lc of each layer can be determined byconverting from the light and shade of the electron diffraction diagramsof TEM (Transmission Electron Microscope) exemplified in FIG. 2 to thedistribution curves as shown in FIG. 3, and calculating Lc using ahalf-value width of each peak. For example, a crystal size can becalculated as a relative value of the known Lc of T300 (carbon fibersupplied by Toray Industries, Inc.). In FIG. 2, a portion appearing in arod-like form is a shade of a measuring device.

Furthermore, so that a core becomes in a softer condition, anorientation degree f of the core is preferably 0.7 or less, and morepreferably 0.6 or less.

By forming such a structure, a high degree of elongation of a carbonfiber can be achieved. The reason of a high degree of elongation issupposed in that, by forming an intermediate layer as a hard layer,relatively soft sheath and core take charge of impact caused when theintermediate layer is broken, and the carbon fiber elongates withoutreaching breakage.

A high degree of elongation of a carbon fiber means one in a range of1.1% or more and 2.5% or less, more preferably in a range of 1.2 to2.5%, and particularly preferably in a range of 1.3 to 2.5%. To thecontrary, a low degree of elongation means one of 1.0% or less. Thehigher the degree of elongation, the better molding processing propertybecomes, thereby suppressing occurrences of fluff in the process ofobtaining a final product.

Next, the thicknesses of the respective layers in the carbon fiber willbe explained. It is preferred that the core occupies 10 to 70% relativeto the cross-sectional area of the fiber, the thickness of the sheath is100 nm to 10,000 nm in a direction perpendicular to the fiber axis so asto cover the core, and the thickness of the intermediate layer is morethan 0 nm and 5,000 nm or less. More preferably, the thickness of theintermediate layer is 100 nm to 5,000 nm. Further, it is preferred thatthe core occupies 30 to 50% relative to the cross-sectional area of thefiber.

A flame resistant fiber is liable to be flattened in section at aninitial stage of carbonization, and tends to become a fiber bundleintermingled with flat yarns. By flattening, because the surface area ofthe fiber increases, the fiber bundle easily radiates heat, and the timefor stabilization tends to be able to be shortened. The cross-sectionalshape of a fiber can be observed by a laser microscope. The rate ofinterminglement of flat yarns was determined by counting the numbers ofnon-circular ones and circular ones, respectively, in a photograph takenat 1,000 times in magnification of a section of a fiber bundle using alaser microscope. Counting was performed by referring a single yarn witha ratio of a minor axis to a major axis of 1 to 0.8 as a circular one,and a single yarn with a ratio of a minor axis to a major axis of 0.1 ormore and less than 0.8 as a flat yarn.

Next, several characteristics of the production method to obtain acarbon fiber will be raised.

In the carbon fiber, because it is possible to obtain a carbonized yarnhaving a sheath-core structure with three or more layers by wet spinninga single kind of polymer and burning it, there is merit in that it isnot necessary to perform compounding, coating or the like, afterspinning. Further, because the respective layers are strongly combinedby performing spinning and burning and forming three or more layers froma single kind of polymer, achieved is a structure in which it ispossible to supplement poor points of the layers each other, asaforementioned.

Next, a polymer solution for spinning will be described. The polymersolution for spinning is preferred to be a polymer prepared by modifyingPAN with an amine-based compound and oxidizing it with a nitro compound.

By using a polymer solution for spinning not containing a nitrocompound, it tends to become possible that an exothermic reaction instabilization of a spun fiber is suppressed, thereby realizingstabilization of the fiber within a shorter period of time. Furthermore,by using a polymer solution for spinning not containing a nitrocompound, because nitrobenzene does not exist in the spun coagulatedyarn and/or dried fiber, it is possible to form a carbon fiber having athree-layer structure through stabilization and carbonization. When anitro compound is left in the polymer solution for spinning, it issupposed that the nitro compound in the fiber operates as an oxidanteven in the process of stabilization, and it is believed that thisoxidation during formation of a structure of a fiber is a cause ofbecoming a carbon fiber with a two-layer structure. As a method ofcontrolling an amount of the nitro compound left in a polymer solutionfor spinning to 0%, there are two kinds of methods of a method ofremoving it by washing with ethanol after PAN is modified with anamine-based compound and a nitro compound, and a method of making anitro compound easily react by increasing the amount of amine-basedcompound. Since washing takes time and incurs cost and there is apossibility of being left in the polymer, more preferred is the lattermethod of controlling the amount of the residual nitro compound to 0% inthe reaction system. Concrete explanation of such a method will bedescribed later.

In PAN composed of only acrylonitrile, a long period of time is requiredfor stabilization of a fiber after spinning, further, burning and fusionand the like are caused during stabilization of the fiber, and theproperties of a carbon fiber finally made tend to be lowered.

As a state “modified with an amine-based compound” referred here,exemplified is a state where an amine-based compound is chemicallyreacted with PAN as a raw material, or a state where an amine-basedcompound is incorporated into a polymer by hydrogen bonding or aninteraction such as van der Waals force.

It is determined by the following methods whether a polymer for spinningis modified with an amine-based compound or not.

A. Method of analyzing a difference in structure with a polymer which isnot modified, by spectroscopic manner, for example, using NMR spectrum,infrared absorption (IR) spectrum or the like aforementioned.B. Method of determining masses of a polymer before and after making apolymer for spinning by a method described later and confirming whetherthe mass of the polymer for spinning is increased relative to the massof PAN as a raw material or not.

In the former method, a section originating from an amine-based compoundused as a modifier is added as a new spectrum in a spectrum of a polymerfor spinning modified with the amine-based compound, relative to aspectrum of PAN as a raw material.

The mass of a polymer for spinning modified with an amine-based compoundincreases by 1.1 times or more, preferably 1.2 times or more,particularly preferably 1.3 times or more, relative to PAN as a rawmaterial. Further, in an increase, the upper limit is preferably 3 timesor less, more preferably 2.6 times or less, and further preferably 2.2times or less. If the change in mass is smaller or greater than such arange, there is a possibility that the spinning property is damaged andthe strength or the degree of elongation of a carbon fiber is reduced.

As an amine-based compound capable of being used to modify a polymer forspinning, although any of compounds having primary to quaterary aminogroup may be employed, concretely, polyethylene polyamines such asethylene diamine, diethylene triamine, triethylene tetramine,tetraethylene pentamine, pentaethylene hexamine and N-aminoethylpiperazine, and ortho, meta and para phenylene diamines can beexemplified.

In particular, it is also preferred to have a functional group having anelement of oxygen, nitrogen, sulfur or the like such as a hydroxyl groupexcept an amino group, and it is preferably a compound having two ormore functional groups including an amino group and such a functionalgroup except the amino group, from the viewpoint of reactivity and thelike. Concretely, ethanol amine group such as monoethanol amine,diethanol amine, triethanol amine and N-aminoethyl ethanol amine can beexemplified. Among these, in particular, monoethanol amine is morepreferred. These can be used solely or at a combination of two or morekinds. In a compound having a functional group except an amino group,for example, having a hydroxyl group, there is a possibility that thehydroxyl group modifies a polymer for spinning.

The nitro compound is an oxidant and oxidizes PAN. Therefore, the fiberspun using PAN modified with an amine and oxidized by a nitro compoundtends to be able to be finished with stabilization in a very shortperiod of time of 10 seconds or more and 15 minutes or less. As thenitro compound, concretely, an oxidant of nitro-based, nitroxide-basedor the like can be exemplified. Among these, as particularly preferableones, aromatic nitro compounds such as nitrobenzene, o, m,p-nitrotoluene, nitroxylene, o, m, p-nitrophenol and o, m,p-nitrobenzoic acid can be exemplified. In particular, nitrobenzenehaving a simple structure is most preferably used, since it is little inrisk, and a quick oxidation is possible because of less sterichindrance.

Although the amount to be added of these oxidants is not particularlyrestricted so that PAN is sufficiently oxidized, it is preferred to usea nitro compound at 10 wt % or more relative to PAN, more preferably 15wt % or more. Further, as the amount to be added of a nitro compound, tocontrol the remaining rate of the nitro compound in the aforementionedpolymer solution for spinning at 0%, it is preferred to use 1 to 50parts by mass relative to 100 parts by mass of an amine-based compoundto be employed. It is more preferred to use 20 to 45 parts by mass. Atthat time, the reaction temperature is preferably 130 to 300° C., andmore preferably 130 to 250° C. The reaction time is preferably 4 hoursor more and 10 hours or less, and more preferably 5 hours or more and 8hours or less. If heated for a time more than 10 hours, a polymer is toodamaged, and finally the strength of a carbon fiber is reduced. In atime less than 4 hours, the nitro compound is liable to be left in thesystem, the structure of a carbon fiber finally obtained does not becomethree layers, and the degree of elongation tends to be reduced.

When PAN is modified under a condition present with an amine-basedcompound after being dissolved in a polar organic solvent, theamine-based compound and the polar organic solvent and an oxidant may bemixed before addition of PAN and may be simultaneously with addition ofPAN. It is preferred that first PAN, an amine-based compound and a polarorganic solvent are mixed, and after dissolution by heating, a polymerfor spinning is prepared by adding an oxidant, from the viewpoint ofless insoluble substances. Of course, it is not obstructed to mix acomponent other than PAN, an oxidant, an amine-based compound and apolar organic solvent with such a solution.

In the polymer for spinning, inorganic particles such as alumina orzeolite, a pigment such as carbon black, an antifoaming agent such assilicone, stabilizer•flame retardant such as a phosphorus compound,various kinds of surfactants, and other additives may be contained.Further, for the purpose of improving the solubility of a polymer forspinning, an inorganic compound such as lithium chloride or calciumchloride can be contained. These may be added before expediting thereaction, and may be added after expediting the reaction.

Further, the molecular weight and the shape of a polymer for spinningare determined by GPC, and it is preferred that the value of thegradient a (hereinafter, referred to as “a”) is 0.1 to 0.3. The “a”determined by GPC means “a” represented by Mark Houwink-Sakuradaequation (1).

[η]=KMw^(a)  (1)

wherein [η] is an intrinsic viscosity, K is a constant inherent for amaterial, and Mw is a weight average molecular weight.

It is known that a polymer exists in a polymer solution as a rod-likepolymer as the value of this gradient “a” is closer to 2, as a randomcoil-like polymer as closer to 0.7, and as a spherical polymer as closerto 0.

It is preferred that the “a” of a polymer for spinning is 0.1 to 0.3,and it is understood that the polymer for spinning becomes a divergentstructure much closer in shape to a spherical shape than to a rod-likeshape. By employing a divergent structure, molecules are moreintertwined with each other as compared to employing a straight-chainstructure. Accordingly, when stabilization of a spun fiber is performed,molecules of the polymer are easily combined with each other, and thetime for the stabilization of the fiber tends to be able to beshortened. Therefore, when the “a” exceeds 0.3, the stabilizationbecomes insufficient, there is a tendency to be decomposed in acarbonization process and a tendency that the differences between the“Lc”s and between the orientation degree “f”s of three layers of acarbon fiber are smallened and the degree of elongation is reduced.Further, when the “a” becomes less than 0.1, because the molecularweight itself is being greatly decreased, spinning becomes difficult.Further, even if spinning can be carried out, the strength of the fibertends to be fairly reduced.

Next, PAN as a raw material will be explained.

PAN may be a homo PAN and may be a copolymerized PAN. With thecopolymerized PAN, from the viewpoint of the solubility of a polymer andthe flame resistant property of a fiber, the structural unit originatingfrom acrylonitrile (hereinafter, referred to as AN) is preferably 85 mol% or more, more preferably 90 mol % or more, and further preferably 92mol % or more.

As concrete copolymerization components, allyl sulfonic acid metal salt,methallyl sulfonic acid metal salt, acrylic ester, methacrylic ester,acrylic amide and the like can be also copolymerized. Further, exceptthe above-described copolymerization components, as components foraccelerating stabilization, components containing a vinyl group,concretely, acrylic acid, methacrylic acid, itaconic acid and the like,can also be copolymerized, and a part or the whole amount thereof may beneutralized with an alkali component such as ammonia.

Further, in PAN as a raw material, it is preferred that the “a”determined by GPC is 0.4 or more and 0.7 or less.

When PAN is dissolved in a polar organic solvent, the shape and form ofthe PAN may be any of powder, flake and fiber, and polymer waste, yarnwaste and the like generated during polymerization or at the time ofspinning can also be used as recycled raw material. Desirably, it ispreferred to be in a form of powder, in particular, microparticles of100 μm or less, from the viewpoint of solubility into solvent.

The polymer solution for spinning can be made by dissolving a polymerfor spinning in an organic solvent. With respect to the concentration ofthe polymer solution for spinning, when the concentration is low,productivity at the time of spinning tends to be low although the effectdue to our method itself is not damaged, and when the concentration ishigh, flowability is poor and it tends to be hard to be spun. Inconsideration of being served to spinning, it is preferably 8 to 30 mass%. The concentration of the polymer for spinning can be determined bythe following method.

The polymer solution for spinning is weighed, the solution of about 4 gis put into distilled water of 500 ml and boiled. A solid material isonce taken out, it is again put into distilled water of 500 ml andboiled. A residual solid component is placed on an aluminum pan, driedfor one day by an oven heated at a temperature of 120° C., and a polymerfor spinning is isolated. The isolated solid component is weighed, andthe concentration is determined by calculating a ratio with the mass ofthe original polymer solution for spinning.

Further, the polymer for spinning tends to be easily made into asolution when employing, in particular, a polar organic solvent as thesolvent among organic solvents. This is because the polymer for spinningmodified with an amine-based compound is high in polarity and thepolymer is well dissolved by a polar organic solvent.

The polar organic solvent means a solvent having an amino group, anamide group, a sulfonyl group, a sulfone group and the like and furtherhaving a good compatibility with water, and as concrete examples,ethylene glycol, diethylene glycol, triethylene glycol, a polyethyleneglycol having a molecular weight of about 200 to 1,000, dimethylsulfoxide (hereinafter, also abbreviated as DMSO), dimethyl formamide,dimethyl acetamide, N-methyl pyrrolidone and the like can be used. Thesemay be used solely, and may be used as a mixture of two or more kinds.In particular, DMSO is preferably used because of its highdissolvability relative to PAN.

The viscosity of the polymer solution for spinning can be set inrespective preferable ranges depending upon a forming method or amolding method using the polymer, a molding temperature, a kind of a dieor a mold and the like. Generally, it can be 1 to 1,000 Pa·s in themeasurement at 50° C. More preferably, it is 10 to 100 Pa·s, and furtherpreferably, it is 20 to 600 Pa·s. Such a viscosity can be measured byvarious viscosity measuring devices, for example, a rotary-typeviscometer, a rheometer, a B-type viscometer or the like. The viscositydetermined by any one method may be controlled in the above-describedrange. Further, even if out of such a range, by heating or cooling atthe time of spinning, it can be used as an appropriate viscosity.

As the method of obtaining a polymer solution for spinning, thefollowing methods are exemplified.

A. A method of modifying PAN with an amine and oxidizing with a nitrocompound in a solution as described above.B. A method of isolating PAN modified with an amine and oxidized with anitro compound, and directly dissolving it in a solvent.

In directly dissolving PAN spun after modification and oxidation in anorganic solvent, the dissolution may be performed under an atmosphericpressure, and as the case may be, it may be performed under apressurized or pressure-reduced condition. As an apparatus used for thedissolution, except a usual reaction vessel with an agitator, a mixersuch as an extruder or a kneader can be used solely or at a form ofcombination thereof.

In this case, the dissolution is preferably performed using anamine-based compound and a polar organic solvent at the sum thereof of100 to 1,900 parts by mass, preferably 150 to 1,500 parts by mass,relative to 100 parts by mass of an acrylic-based polymer.

Although it is preferred that non-reacted substances, insolublesubstances, gel and the like are not contained in the polymer solutionfor spinning obtained by the above-described method, there is apossibility that they are left at a fine amount. It is preferred tofiltrate or disperse non-reacted substances or insoluble substancesusing a sintered filter or the like before formation into fibers.

Next, the method of producing a flame resistant fiber suitable to obtaina carbon fiber will be explained.

As the method of spinning the polymer solution for spinning into afiber, a wet spinning or a dry/wet spinning is employed to improveproductivity of the process. Preferably, a wet spinning is used.

Concretely, the spinning can be performed by preparing theaforementioned polymer solution for spinning as a polymer solution forspinning, elevating the pressure through a pipe by a booster pump or thelike, extruding with metering by a gear pump or the like, anddischarging from a die. As the material of the die, SUS (stainless),gold, platinum and the like can be appropriately used.

Further, it is preferred that, before the polymer solution for spinningflows into holes of the die, the polymer solution for spinning isfiltrated or dispersed using a sintered filter of inorganic fibers orusing a woven fabric, a knitted fabric, a nonwoven fabric or the likecomprising synthetic fibers such as polyester or polyamide as a filter,from the viewpoint that the fluctuation of the cross-sectional areas ofsingle fibers in a fiber aggregate to be obtained can be reduced.

As the hole diameter of the die, an arbitrary range of 0.01 to 0.5 mmφcan be employed, and as the hole length, an arbitrary range of 0.01 to 1mm can be employed. Further, as the number of the holes of the die, anarbitrary range of 10 to 1,000,000 can be employed. As the holearrangement, an arbitrary one such as a staggered arrangement can beemployed, and the holes may be divided in advance so as to realize easyyarn dividing.

Coagulated yarns are obtained by discharging the polymer solution forspinning from the die directly or indirectly into a coagulation bath. Itis preferred that the liquid for the coagulation bath is formed from asolvent used for the polymer solution for spinning and a coagulationacceleration component, from the viewpoint of convenience, and it ismore preferred to use water as the coagulation acceleration component.Although the rate of the solvent for spinning to the coagulationacceleration component in the coagulation bath and the temperature ofthe liquid for the coagulation bath are appropriately selected and setin consideration of denseness, surface smoothness, spinnability and thelike of the coagulated yarns to be obtained, in particular, as theconcentration of the coagulation bath, an arbitrary concentration can beemployed within a range of solvent/water=0/100 to 95/5, and 30/70 to70/30 is preferable, and 40/60 to 60/40 is particularly preferable.Further, as the temperature of the coagulation bath, an arbitrarytemperature of 0 to 100° C. can be employed. Further, as the coagulationbath, if an alcohol such as propanol or butanol reducing an affinitywith water is employed, it can also be used as 100% bath.

In the method of producing a carbon fiber, the degree of swelling of thecoagulated yarn obtained is preferably controlled to 50 to 1,000 mass %,more preferably 200 to 900 mass %, and further preferably 300 to 800mass %. The degree of swelling of the coagulated yarn controlled in sucha range greatly relates to the toughness and easiness in deformation ofthe coagulated yarn and affects the spinnability. The degree of swellingis decided from the viewpoint of spinnability, and affects a stretchingproperty in bath at a later process, and if in such a range, thecoefficient of variation of the cross-sectional areas of single fiberscan be made small in the carbon fibers to be obtained. The degree ofswelling of the coagulated yarn can be controlled by the affinitybetween the polymer for spinning forming the coagulated yarn and thecoagulation bath and the temperature or the concentration of thecoagulation bath, and a degree of swelling in the above-described rangecan be achieved by controlling the temperature of the coagulation bathor the concentration of the coagulation bath in the aforementioned rangerelative to a specified polymer for spinning.

Next, it is preferred that the coagulated yarn is stretched in astretching bath or washed in a water washing bath. Of course, it may bestretched in a stretching bath as well as washed in a water washingbath. The draw ratio for the stretching is preferably 1.05 to 5 times,more preferably 1.1 to 3 times, and further preferably 1.15 to 2.5times. For the stretching bath, hot water or solvent/water is used, andthe concentration of solvent/water for the stretching bath can be set atan arbitrary concentration of 0/100 to 80/20. Further, for the waterwashing bath, usually hot water is used, and the temperature of both thestretching bath and the water washing bath is preferably 30 to 100° C.,more preferably 50 to 95° C., and particularly preferably 65 to 95° C.

The fiber completed with coagulation is dried, and as needed, stretchedto become a carbon fiber through stabilization and carbonization.

As the drying method, a drying method of bringing the fiber into directcontact with a plurality of dried and heated rollers, a drying method ofsending hot air or water vapor, a drying method of irradiating infraredrays or electromagnetic rays with a high frequency, a drying method ofmaking a pressure reduced condition or the like can be appropriatelyselected and combined. Usually, in a drying method due to hot air, hotair is sent in a direction parallel or perpendicular to the runningdirection of the fiber. For the infrared rays of radiation-heating type,far infrared rays, mid infrared rays or near infrared rays can beemployed, and radiation of microwaves can also be employed. Although thetemperature for the drying can be employed arbitrarily at approximately50 to 250° C., generally, the drying takes a long time at a lowtemperature and a short time at a high temperature.

When stretching is carried out after drying, the specific gravity of thefiber after drying is usually 1.15 to 1.5, preferably 1.2 to 1.4, andmore preferably 1.2 to 1.35. The coefficient of variation of thecross-sectional areas of single fibers in the fiber aggregate afterdrying is preferably 5 to 30%, more preferably 7 to 28%, and furtherpreferably 10 to 25%. Further, elongation of the single fiber in thefiber aggregate after drying is preferably 0.5 to 20%. Furthermore, inthe fiber aggregate after drying, oxidation calorific value (J/g)determined by differential scanning calorimetry (DSC) is preferably 50to 4,000 J/g. As the case may be, not a continuous drying but a batchdrying can be carried out.

For such a stretching process, because a fiber is plasticized withmoisture, it is preferred to use a method of heating the fiber at acondition of containing water in the fiber such as a bath stretchingusing warm water or hot water, a stretching using steam (water vapor),or a heat stretching by a dryer or rolls after providing water to thefiber in advance, and heating/stretching by steam stretching isparticularly preferred.

In bath stretching, it is preferred that the stretching is carried outat a temperature of, preferably 70° C. or higher, more preferably 80° C.or higher, and further preferably 90° C. or higher. At this stage, thefiber structure is already densified even if the temperature iselevated, there is no fear of generating micro voids, and a stretchingat a temperature as high as possible is preferred because a high effectdue to molecular orientation can be obtained. Although it is preferredto use water for the bath, the stretching property may be furtherenhanced by adding a solvent or other additives.

Although a higher stretching temperature is preferred, in a bathstretching, basically 100° C. becomes the upper limit. Accordingly, astretching using steam is employed more preferably. Although thetemperature of the stretching is preferred to be higher, when asaturated vapor is used, because the internal pressure of the apparatusis high, there is a possibility that the fiber is damaged by blowingvapor. For the purpose of obtaining a carbon fiber with a degree oforientation of the sheath of 65% or more, a saturated vapor with atemperature of 100° C. or higher and 150° C. or lower may be used. Ifthe temperature exceeds 150° C., the effect due to the plasticizationgradually gets to the top, and damage of the fiber due to blowing vaporbecomes greater than the effect due to the plasticization. As thestretching treatment apparatus using a saturated vapor, an apparatusdevising to pressurize the inside of the treatment apparatus byproviding a plurality of apertures at the fiber inlet and outlet ispreferably used.

It is also possible to use a super-heated atmospheric high-temperaturesteam to prevent the damage of the fiber due to blowing vapor. Thisbecomes possible by heating an atmospheric steam using electric heating,water vapor heating, induction heating or the like and, thereafter,introducing it into the stretching treatment apparatus. Although it ispossible to employ a range of 100° C. or higher and 170° C. or lower forthe temperature, it is preferred to be 110° C. or higher and 150° C. orlower. If the temperature is too high, the moisture contained in thesteam is reduced, and the effect of plasticizing the fiber becomes hardto be obtained.

The draw ratio for the bath stretching and the draw ratio for thestretching by steam are preferably 1.5 times or more, and morepreferably 2.0 times or more. To promote the molecular orientation, thedraw ratio for the stretching is preferred to be higher, and an upperlimit thereof is not particularly present. However, from restriction onstability of spinning, it is frequently difficult to exceed about 6times.

Further, in the method of stretching the fiber, the means thereof is notrestricted to the bath stretching or the steam stretching. For example,heat stretching by a drying furnace or a hot roller or the like afterproviding moisture may be possible.

A non-contact type stretching machine using a drying furnace, further, acontact type stretching machine using a contact plate, a hot roller orthe like, can also be used. However, in a contact type stretchingmachine, evaporation of moisture is fast and, further, there is a highpossibility that a fiber is mechanically scratched at a point occurredwith stretching. Further, in a non-contact type stretching machine, arequired temperature becomes 250° C. or higher, and as the case may be,thermal decomposition of the polymer starts. Furthermore, when anon-contact type stretching machine or a contact type stretching machineis used, the effect due to stretching is low, and it is more difficultto obtain a carbon fiber with a high orientation than the stretchingmethod using moisture. From these reasons, it is more preferred to use abath stretching or a steam stretching.

The stretched yarn thus stretched is preferably dried again, as needed.The moisture percentage of the fiber is preferably 10% or less, and morepreferably 5% or less. As this drying method, bringing the fiber intocontact directly with a plurality of dried and heated rollers or hotplates, sending hot air or water vapor, irradiating infrared rays orelectromagnetic rays with a high frequency, making a pressure reducedcondition and the like can be appropriately selected and combined. It ispreferred to employ drying due to rollers to perform an efficientdrying. The number of the rollers is not restricted. The temperature ofthe rollers is preferably 100° C. or higher and 250° C. or lower, andmore preferably 150° C. or higher and 200° C. or lower. If the drying atthis process is insufficient, there is a possibility to cause a fiberbreakage when a tension is applied to the fiber at a heat treatmentprocess carried out later.

To the coagulate yarn, or the fiber at a water swelling state afterbeing water washed and stretched, an oil component can be appropriatelyprovided depending upon the necessity of a higher-order processing. Whenan oil component is provided, usually the concentration of the oil isset at 0.01 to 20 mass %. As the method of providing, it may beappropriately selected and employed in consideration of being provideduniformly up to the interior of the yarn. Concretely, a method such asdipping of the yarn into an oil bath or spray or dropping onto therunning yarn is employed. The oil comprises, for example, a main oilcomponent such as silicone and a diluent component for diluting it. Theconcentration of oil means a content of the main oil component relativeto the whole of the oil. The kind of the oil component is notparticularly restricted, polyether-based one, polyester surfactant,silicone, amino-modified silicone, epoxy-modified silicone orpolyether-modified silicone can be provided solely or at a mixturethereof, and other oil components may be provided.

The adhesion amount of such an oil component is determined as a raterelative to the dried mass of the fiber included with the oil component,and it is preferably 0.05 to 5 mass %, more preferably 0.1 to 3 mass %,and further preferably 0.1 to 2 mass %. If the adhesion amount of an oilcomponent is too little, there is a possibility that fusion of singlefibers to each other occurs and the tensile strength of an obtainedcarbon fiber is reduced and, if too much, there is a possibility that itbecomes difficult to obtain the desired effect.

The fiber obtained by the above-described process is transferred to aprocess for stabilization. The fiber before being transferred to thestabilization process is preferably in a dried condition. As the methodof stabilization, in particular, it is preferred to use a dry-heatingapparatus to control chemical reaction and suppress unevenness in fiberstructure, and concrete equipment thereof will be described later. Thetemperature and the treatment length are appropriately selecteddepending upon the oxidation degree of the used polymer for spinning,the fiber orientation degree and the required properties for a finalproduct. Concretely, the treatment temperature for the stabilization ispreferably 280° C. or higher and 400° C. or lower. More preferably, itis 300° C. or higher and 360° C. or lower, and particularly preferably,it is 300° C. to 330° C. If the temperature is lower than 280° C., aproblem tends to occur in a carbonization process. If the temperatureexceeds 400° C., the fiber tends to be decomposed in a stabilizationfurnace. The treatment time of the stabilization is preferably 10seconds or longer to prevent decomposition in a carbonization process.Further, when the treatment time of the stabilization exceeds 15minutes, because the merit of shortening the time for stabilizationbecomes small and besides the fiber is fuzzed to cause reduction ofstrength and degree of elongation, it is preferred that the treatmenttime of the stabilization is 15 minutes or shorter. From the viewpointof suppressing occurrences of fluff, more preferably it is 5 minutes orshorter.

Further, it is preferred to perform a stretching when the heat treatmentis carried out. By carrying out the stretching treatment, the molecularorientation can be further enhanced. The draw ratio for this stretchingis preferably 1.05 to 4 times. The draw ratio is set from requiredstrength and fineness of the flame resistant fiber, processpassing-through property and the temperature of the heat treatment.Concretely, the draw ratio for the stretching is 1.1 to 4 times,preferably 1.2 to 3 times, and more preferably 1.3 to 2.5 times.Further, it is also important to perform heat treatment at the time ofstretching, and as the time for the heat treatment, an arbitrary valueof 1 to 15 minutes can be employed depending upon the temperature.Stretching and treatment for stabilization may be performed eithersimultaneously or separately.

Among dry-heating apparatuses, in particular, it is preferred to use aninfrared heater and a hot air drier together. By employing heating dueto an infrared heater and a hot air drier together, the treatment timefor stabilization tends to be shortened.

To use an infrared heater and a hot air drier together includes to treatseparately from each other, and it is particularly preferred to providean infrared heater in a hot air circulation drier and performsimultaneous treatment of emission (radiation) and heat transfer by theintegrated hot air circulation drier equipped with the infrared heater.By using the integrated apparatus, high temperature-elevation•short-timetreatment due to the infrared heater and uniform treatment of singlefibers due to hot air can be achieved simultaneously. Although a metal,a ceramic or the like can be used as the material of the infraredheater, it is preferred to be made from a ceramic from its high heatradiation rate and high thermal stability.

A schematic structure of a hot air circulation drier equipped with aninfrared heater is exemplified in FIG. 4, and as shown in the figure, itcan be manufactured, for example, by providing two or more openings 15a, 15 b to a forced-type hot air circulation drier 11 sold on the marketso as to be able to treat a fiber continuously and, further, attachingan electric ceramic heater 16 sold on the market (for example, a ceramicplate heater “PLC-323”, supplied by NORITAKE CO., LTD.) inside thedrier. It is preferred that two or more ceramic heaters are installedand, further, it is particularly preferred that they are installed to beable to irradiate the infrared rays to the fiber from both directions ofupper and lower sides or left and right sides to irradiate the infraredrays to the fiber uniformly. With respect to the treatment by the hotair circulation drier 11, for example, a non-treated fiber 12 (fiberbefore treatment) is introduced into hot air circulation drier 11 fromopening 15 a while being guided by a roller 14 a, it is irradiated withthe infrared rays from both directions of upper and lower sides byceramic heaters 16 attached to, for example, punching metals 17 forattaching ceramic heaters, and at the same time, heat transfer treatmentdue to hot air (the flow of the hot air is shown by arrows 18) isperformed, and a stabilized fiber 13 (fiber after treatment) is sent outfrom opening 15 b while being guided by a roller 14 b.

As the circulation system of the hot air circulation drier, both a downflow system and an up flow system can be applied. As a fan to controlthe circulation amount of hot air, although a propeller fan and asirocco fan can be used, it is preferred to use a sirocco fan from theviewpoint of its good wind resistance. It is preferred to rotate thisfan by a motor after conversion to a direct current by an inverter. As aconcrete inverter, “FR-E720-0.2K” supplied by Mitsubishi ElectricCorporation can be exemplified, and as an induction motor, “5IK60A-SF”supplied by ORIENTAL MOTOR Co., Ltd. can be exemplified. Further, as therotational speed of the fan, it is preferably 500 to 1,500 rpm, and toshorten the treatment time within a range which does not cause to fuzz,particularly preferably it is 800 to 1,200 rpm.

Furthermore, by suppressing exothermic reaction at the time ofstabilization, it is possible to shorten the treatment time forstabilization and perform stabilization, which has been performed by twofurnaces, by a single furnace.

The fibers having been spun are in a bundle form comprising a pluralityof single fibers, the number of single fibers included in a singlebundle can be appropriately selected depending upon the purpose of useand, to control the aforementioned preferred number, it can be adjustedby the number of holes of a die, and a plurality of spun fibers may bedoubled.

Further, to control the fineness of the single fiber in theaforementioned preferable range, it can be controlled by selecting thehole diameter of a die or appropriately deciding the discharge amountfrom a die.

Further, when the fineness of a single fiber is made greater, making thetime for drying longer, or elevating the temperature for drying higher,is preferred from the viewpoint of reduction of the amount of residualsolvent.

Further, the cross-sectional shape of a single fiber can be controlledby the shape of a discharge hole of a die such as a circular hole, anoval hole or a slit and the condition at the time of removing a solvent.

Next, a production method suitable to obtain a carbon fiber using theobtained flame resistant fiber will be explained.

A carbon fiber is obtained by heat treating the flame resistant fiber ata high temperature in an inert atmosphere, so-called carbonizing. As aconcrete method of obtaining a carbon fiber, a carbon fiber can beobtained by treating the aforementioned flame resistant fiber at ahighest temperature in an inert atmosphere of 1,000° C. or higher andlower than 2,000° C. More preferably, as the lower side of the highesttemperature, 1,000° C. or higher, 1,200° C. or higher and 1,300° C. orhigher are preferred in order, and as the upper side of the highesttemperature, 1,800° C. or lower can also be employed. Further, byfurther heating such a carbon fiber in an inert atmosphere at atemperature of 2,000 to 3,000° C., a carbon fiber developing in graphitestructure can also be obtained.

In the carbon fiber, the density is preferably 1.6 to 1.9 g/cm³, andmore preferably 1.7 to 1.9 g/cm³. If such a density is too small, thereis a possibility that many pores are present in a single fiber and thefiber strength is reduced, and on the contrary, if too great, there is apossibility that the denseness becomes too high and the degree ofelongation is reduced. Such a density can be determined utilizingimmersion method or sink-float method based on JIS R 7603(1999).

Usually, the single fibers of the carbon fibers are gathered to form anaggregate such as a fiber bundle. In forming the fibers as a bundle,although the number of single fibers per one bundle is appropriatelydecided depending on the purpose of use, from the viewpoint ofhigher-order processing property, it is preferably 50 to 100,000/bundle,more preferably 100 to 80,000/bundle, and further preferably 200 to60,000/bundle.

The tensile strength of a single fiber is preferably 1.0 to 10.0 GPa,more preferably 1.5 to 7.0 GPa, and further preferably 2.0 to 7.0 GPa.Such a tensile strength can be determined based on JIS R7606(2000) usinga universal tensile testing machine (for example, small-sized desk-toptester EZ-S, supplied by Shimadzu Corporation).

It is desired that the diameter of the single fiber is 2 μm or more, inparticular, 2 μm to 70 μm, preferably 2 to 50 μm, and more preferably 3to 20 μm. If such a diameter of the single fiber is less than 2 μm,there is a possibility that the fiber is liable to be broken, and ifmore than 70 μm, a defect rather tends to be caused. The single fiber ofthe carbon fiber may be one having a hollow portion. In this case, thehollow portion may be either continuous or discontinuous.

From the viewpoint of reducing cost, it is preferred to produce a carbonfiber continuously by one process from a polymer for spinning to thecarbon fiber.

The carbon fiber tends to have a peak nearly at 26° in X-ray diffraction(XRD) similarly in a general PAN-based carbon fiber.

EXAMPLES

Next, our fibers and methods will be explained more concretely byExamples. In the Examples, the respective properties and characteristicswere determined by the following methods.

Preparation of Polymer Solutions for Spinning (a, c to e)

A thermometer, a cooler, an agitator and a nitrogen introducing tubewere attached to a three neck flask having a sufficient capacity. Inthis flask, PAN was dissolved in DMSO at the rate described in Table 1,an amine-based compound and a nitro compound were added, and whilestirring by a stirring blade at 300 rpm, heating was carried out in anoil bath at 150° C. for the time described in Table 1 to perform areaction.

Preparation of Polymer Solution for Spinning (b)

PAN and DMSO were put into a polyethylene bottle of 2 L, and they werestirred at 80° C. for the time described in Table 1 to dissolve PAN.

Isolation of Polymer for Spinning

The obtained polymer solution for spinning was washed with ethanol orhot water, and the precipitate was dried to obtain a polymer forspinning.

Spinning

By the above-described method, the obtained polymer solution forspinning was served to a wet spinning apparatus as it was, therebyforming fibers. The dried fiber was 1 denier.

Determination of Molecular Weight by GPC

It was dissolved in N-methyl pyrrolidone (added with 0.01N-lithiumbromide) so that the concentration of a polymer for spinning to bedetermined became 2 mg/mL to prepare a specimen solution. With respectto the prepared specimen solution, a distribution curve of the absolutemolecular weight was determined from the GPC curve measured at thefollowing conditions using a GPC apparatus, and a weight averagemolecular weight Mw was calculated. The measurement was carried out atn=1.

-   -   GPC apparatus: PROMINAICE (supplied by Shimadzu Corporation)    -   Column: polar organic solvent-system GPC column TSK-GEL-α-M (×2)        (supplied by Tosoh Corporation)    -   Detector: (viscosity detection and R1 detection system) Viscotek        Model 305TDA Detectors (supplied by Malvern Corporation)    -   Flow rate: 0.6 mL/min.    -   Temperature: 40° C.    -   Filtration of sample: membrane filter (0.45 μm cut)    -   Amount of injection: 100 μL

Determination of Residual Amount of Nitro Compound by GC-MS

A calibration curve of an added nitro compound was made. The method ofdetermining a sample is as follows.

A polymer extract extracted with ethanol was determined by GC-MS (GasChromatography-Mass Spectroscopy), and compounds present in the extractwere identified by automatic analysis. The measurement was carried outat n=1.

The conditions of the determination of GC-MS are as follows.

-   -   System: GCMS-QP2010 Ultra (supplied by Shimadzu Corporation)    -   Column oven temperature: 500° C.    -   Column flow rate: 1 mL/min.    -   Column: PtxR Amine, film thickness: 1 μm, length: 30 cm, inner        diameter: 0.25 mm GC determination program:    -   Temperature elevation speed: 10° C./min.    -   Range of determination: 50° C. (maintained for 1 min.)→280° C.        (maintained for 1 min.) M/Z (M: mass of molecule, Z: number of        electric charge) determination program:    -   Scanning speed: 1250    -   Starting time: 8 min.    -   Finishing time: 25 min.    -   Scanning speed: 1250    -   Starting m/z: 50    -   Finishing m/z: 400

Stabilization

The treatment was carried out under a condition of air at predeterminedtemperature and temperature elevation speed, using one furnace of a hotair circulation drier incorporated with an infrared heater as shown inFIG. 4. The hot air circulation drier was a down flow-system one, asirocco fan having a diameter of 200 mm was controlled by an inverter(FR-E720-0.2K) supplied by Mitsubishi Electric Corporation and, further,it was rotated by an induction motor (5IK60A-SF) supplied by ORIENTALMOTOR Co., Ltd. The wind direction of the hot air was a cross flow, andthe rotational speed of the fan was 1,200 rpm. Furthermore, as theinfrared heater in the hot air circulation drier, and six electricceramic plate heaters (PLC-323) supplied by NORITAKE CO., LTD. wereinstalled at each of the upper side and the lower side relative to ayarn path, respectively. The temperature of the hot air in the furnaceand the temperature of the infrared heater were set at an identicaltemperature.

Carbonization

The treatment was carried out under a nitrogen atmosphere at apredetermined temperature and at a tensile condition. The carbonizationwas carried out by two furnaces. In the first furnace, the treatment wascarried out at a temperature of 700 to 800° C., and in the secondfurnace, the treatment was carried out at a temperature of 1,300° C. Thetemperature elevation speed was 50 to 200° C.

Determination of Density of Fiber

It was determined based on the sink-float method of JIS R 7603(1999).

Determination of Areal Weight of Fiber Bundle

The mass of a sample cut out by 1 m from 12,000 carbon fibers wasmeasured, and it was determined as the areal weight. The unit of theareal weight is g/m.

Calculation of Diameter of Single Fiber

An average value calculated from the above-described density of fiberand areal weight of fiber bundle by the following equation Equation (1)was calculated as a diameter of a cross section of a single fiber.

$\begin{matrix}\begin{matrix}{l = {\sqrt{\frac{Mf}{120000 \times \rho \times 100 \times \pi}} \times 20000}} \\{= {\sqrt{\frac{Mf}{\rho}} \times 10.3}}\end{matrix} & (1)\end{matrix}$

In the above-described Equation (1), represented are 1: diameter ofsingle fiber (μm), Mf: areal weight of 12,000 carbon fibers (g/m), andρ: density (g/cm³).

Determination of Strength and Degree of Elongation of Single Fiber byTensing Single Fiber

The strength and degree of elongation of a single fiber were determinedunder the following conditions based on JIS R7606 (2000). Further, thestrength was calculated by dividing a maximum load in an S-S curve withthe cross section calculated from the density and the areal weight.Further, the degree of elongation was calculated from a displacement.The number of n was set at 5 or more.

The conditions for the determination are as follows.

-   -   System: small-sized desk-top tester EZ-S (supplied by Shimadzu        Corporation)    -   Load cell: 20N (PEG50NA)    -   Operation for control: loading    -   Testing control: stroke    -   Testing speed: 1 mm/min.    -   Sampling: 50 msec.    -   Free length pace between grippers: 25 mm

TEM Observation

After a specimen was embedded with a resin on a Si base plate, twoprotective layers of Pt-based (conductive treatment) and C-based layerswere deposited. This specimen was chipped in a fiber axis direction bythe following method to prepare a thin-film test piece having athickness of several-hundred μm. Further, it was chipped in parallel tothe fiber axis direction to be able to pick up a center of a fiber,thereby preparing a thin-film test piece having a thickness ofseveral-hundred μm. If hitting a void present in a fiber when a thinfilm for TEM is prepared, a sample is prepared at another position withno voids.

-   -   Method: FIB (Focused Ion Beam)    -   System: SMI3200SE supplied by SINT Corporation, FB-2000A        supplied by Hitachi, Ltd., STRATA400S supplied by FEI        Corporation    -   System: transmission electron microscope; H-9000UHR No. 2        machine supplied by Hitachi, Ltd.    -   Acceleration voltage: 300 kV    -   Diaphragm of restricted visual field: about 300 nmφ        Making of Intensity Distribution Graph and Calculation of        Crystal Size and Orientation Degree from TEM Image

Intensity distribution graph was made from shades of colors by imageanalysis of TEM image. Further, from the intensity distribution graph, acrystal size Lc was calculated from a half-value width of a peakcorresponding to (002) plane by the following equation Equation (2), andan orientation degree of a crystal was calculated from a total width ofa half-value of the intensity distribution in each orientation directionby the following equation Equation (3).

$\begin{matrix}{L_{c} = {\sqrt{\frac{\ln \mspace{11mu} 2}{\pi}}\frac{\lambda}{{\sin \mspace{11mu} \theta_{h}} - {\sin \mspace{11mu} \theta_{l}}}}} & (2)\end{matrix}$

In the above-described equation Equation (2), θh: high angle side of(002) plane, and θl: low angle side of (002) plane.

$\begin{matrix}{f = \frac{180 - {FWHM}}{180}} & (3)\end{matrix}$

In the above-described equation Equation (3), FWHM is a total width of ahalf-value of intensity distribution in each orientation direction.

Elemental Analysis

Measurement was carried out with n number of 2, and an average value ofthese two values was determined as the measured value. However, when adifference between the two values (the respective elemental rates of C,H and N) was more than ±0.4%, the measurement was repeated until itbecame ±0.4% or less.

The conditions for the measurement are as follows.

-   -   System: small-sized elemental analysis device, EuroEA3000        supplied by Evisa Corporation    -   Cup: Tin capsules Pressed 5×9 mm Code E12007    -   Reaction tube: Packed reactor single for CHNS/S 18/6 mm Code        E13040    -   Carrier: 60 kPa    -   Purge: 80 mL/min.    -   Oxygen: 15 mL    -   AP O₂: 35 kPa    -   Oxygen Time: 6.6 sec.    -   Sample Delay: 5 sec.    -   Run Time: 320 sec.    -   Front Furnace: 980° C.    -   Oven: 100° C.

Observation of Fiber Bundle by SEM

SEM determination was carried out at the following conditions.

-   -   System: VK-9800 (supplied by KEYENCE Corporation)    -   Acceleration voltage: 10 kV    -   Spot diameter: 4

Laser Microscope

The observation of a fiber in a laser microscope was carried out at thefollowing conditions.

-   -   System: VK-X210 (supplied by KEYENCE Corporation)    -   Lens: 50× (integrated lens: 20×), observed at a total        magnification of 1,000 times.

Example 1

Polymer solution for spinning (a) was wet spun at a number of filamentsof 12,000 to obtain fibers through a drying process. The obtained fiberswere served to stabilization at conditions of 300° C. and 5 minutes, andcarbonization was carried out at a carbonization temperature of 1,300°C.

As the result of TEM observation, the obtained carbon fiber had a3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at thesheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. Withrespect to orientation degree f, the sheath was oriented at 0.86, theintermediate layer was oriented at 0.89 and the core was oriented at 0.6or less. As the result of tensing a single fiber, the tensile strengthwas 2.1 GPa, the degree of elongation was 1.7%, and they were goodresults.

Example 2

Polymer solution for spinning (a) was treated in a manner similar tothat in Example 1 to obtain fibers. The obtained fibers were served tostabilization. For the obtained fibers, the stabilization was carriedout at conditions of 320° C. and 5 minutes, and carbonization wascarried out at a carbonization temperature of 1,300° C.

As the result of TEM observation, the obtained carbon fiber had a3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at thesheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. Withrespect to orientation degree f, the sheath was oriented at 0.86, theintermediate layer was oriented at 0.89 and the core was oriented at 0.6or less. As the result of tensing a single fiber, the tensile strengthwas 2.1 GPa, the degree of elongation was 1.6%, and they were goodresults.

Example 3

Polymer solution for spinning (a) was treated in a manner similar tothat in Example 1 to obtain fibers. The obtained fibers were served tostabilization. For the obtained fibers, the stabilization was carriedout at conditions of 340° C. and 5 minutes, and carbonization wascarried out at a carbonization temperature of 1,300° C.

As the result of TEM observation, the obtained carbon fiber had a3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at thesheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. Withrespect to orientation degree f, the sheath was oriented at 0.86, theintermediate layer was oriented at 0.89 and the core was oriented at 0.6or less. As the result of tensing a single fiber, the tensile strengthwas 2.2 GPa, the degree of elongation was 1.5%, and they were goodresults.

Example 4

Polymer solution for spinning (a) was treated in a manner similar tothat in Example 1 to obtain fibers. The obtained fibers were served tostabilization. For the obtained fibers, the stabilization was carriedout at conditions of 360° C. and 5 minutes, and carbonization wascarried out at a carbonization temperature of 1,300° C.

As the result of TEM observation, the obtained carbon fiber had a3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at thesheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. Withrespect to orientation degree f, the sheath was oriented at 0.86, theintermediate layer was oriented at 0.89 and the core was oriented at 0.6or less. As the result of tensing a single fiber, the tensile strengthwas 2.2 GPa, the degree of elongation was 1.5%, and they were goodresults.

Example 5

Polymer solution for spinning (a) was treated in a manner similar tothat in Example 1 to obtain fibers. The obtained fibers were served tostabilization. For the obtained fibers, the stabilization was carriedout at conditions of 300° C. and 10 minutes, and carbonization wascarried out at a carbonization temperature of 1,300° C.

As the result of TEM observation, the obtained carbon fiber had a3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at thesheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. Withrespect to orientation degree f, the sheath was oriented at 0.86, theintermediate layer was oriented at 0.89 and the core was oriented at 0.6or less. As the result of tensing a single fiber, the tensile strengthwas 2.2 GPa, the degree of elongation was 1.6%, and they were goodresults.

Example 6

Polymer solution for spinning (a) was treated in a manner similar tothat in Example 1 to obtain fibers. The obtained fibers were served tostabilization. For the obtained fibers, the stabilization was carriedout at conditions of 360° C. and 10 minutes, and carbonization wascarried out at a carbonization temperature of 1,300° C.

As the result of TEM observation, the obtained carbon fiber had a3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at thesheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. Withrespect to orientation degree f, the sheath was oriented at 0.86, theintermediate layer was oriented at 0.89 and the core was oriented at 0.6or less. As the result of tensing a single fiber, the tensile strengthwas 2.4 GPa, the degree of elongation was 1.6%, and they were goodresults.

Example 7

Polymer solution for spinning (a) was treated in a manner similar tothat in Example 1 to obtain fibers. The obtained fibers were served tostabilization. For the obtained fibers, the stabilization was carriedout at conditions of 300° C. and 15 minutes, and carbonization wascarried out at a carbonization temperature of 1,300° C.

As the result of TEM observation, the obtained carbon fiber had a3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at thesheath, 1.8 nm at the intermediate layer and 2.0 nm at the core. Withrespect to orientation degree f, the sheath was oriented at 0.86, theintermediate layer was oriented at 0.89 and the core was oriented at 0.6or less. As the result of tensing a single fiber, the tensile strengthwas 2.3 GPa, the degree of elongation was 1.6%, and they were goodresults.

Example 8

Polymer solution for spinning (a) was treated in a manner similar tothat in Example 1 to obtain fibers. The obtained fibers were served tostabilization. For the obtained fibers, the stabilization was carriedout at conditions of 360° C. and 15 minutes, and carbonization wascarried out at a carbonization temperature of 1,300° C.

As the result of TEM observation, the obtained carbon fiber had a3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at thesheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. Withrespect to orientation degree f, the sheath was oriented at 0.85, theintermediate layer was oriented at 0.88 and the core was oriented at 0.6or less. As the result of tensing a single fiber, the tensile strengthwas 2.4 GPa, the degree of elongation was 1.6%, and they were goodresults.

Example 9

Polymer solution for spinning (d) was treated in a manner similar tothat in Example 1 to obtain fibers. The obtained fibers were served tostabilization. For the obtained fibers, the stabilization was carriedout at conditions of 360° C. and 15 minutes, and carbonization wascarried out at a carbonization temperature of 1,300° C.

As the result of TEM observation, the obtained carbon fiber had a3-layer sheath-core structure. With respect to Lc, it was 1.4 nm at thesheath, 1.6 nm at the intermediate layer and 1.8 nm at the core. Withrespect to orientation degree f, the sheath was oriented at 0.82, theintermediate layer was oriented at 0.84 and the core was oriented at 0.6or less. As the result of tensing a single fiber, the tensile strengthwas 2.0 GPa, the degree of elongation was 1.3%, and they were goodresults.

Example 10

Polymer solution for spinning (e) was treated in a manner similar tothat in Example 1 to obtain fibers. The obtained fibers were served tostabilization. For the obtained fibers, the stabilization was carriedout at conditions of 360° C. and 15 minutes, and carbonization wascarried out at a carbonization temperature of 1,300° C.

As the result of TEM observation, the obtained carbon fiber had a3-layer sheath-core structure. With respect to Lc, it was 1.4 nm at thesheath, 1.6 nm at the intermediate layer and 1.8 nm at the core. Withrespect to orientation degree f, the sheath was oriented at 0.82, theintermediate layer was oriented at 0.84 and the core was oriented at 0.6or less. As the result of tensing a single fiber, the tensile strengthwas 1.6 GPa, the degree of elongation was 1.6%, and they were goodresults.

Example 11

Polymer solution for spinning (a) was treated in a manner similar tothat in Example 1 to obtain fibers. The obtained fibers were served tostabilization. For the obtained fibers, the stabilization was carriedout at conditions of 360° C. and 30 minutes, and carbonization wascarried out at a carbonization temperature of 1,300° C.

As the result of TEM observation, the obtained carbon fiber had a3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at thesheath, 1.8 nm at the intermediate layer and 2.0 nm at the core. Withrespect to orientation degree f, the sheath was oriented at 0.79, theintermediate layer was oriented at 0.81 and the core was oriented at 0.6or less. As the result of tensing a single fiber, because the time forstabilization was too long, the fiber was fuzzed and the thicknessthereof became small and, therefore, the tensile strength was reduced to1.7 GPa, the degree of elongation was reduced to 1.5%, but they weregood results.

Comparative Example 1

Polymer solution for spinning (a) was wet spun in a manner similar tothat in Example 1 to obtain fibers through a drying process. Theobtained fibers were served to stabilization at conditions of 240° C.and 15 minutes. Although the stabilized fiber was tried to be carriedout with carbonization at a carbonization temperature of 1,300° C., thefiber was burned and broken immediately after being introduced into afurnace, and could not be carbonized as a carbon fiber.

Comparative Example 2

Polymer solution for spinning (a) was wet spun in a manner similar tothat in Example 1 to obtain fibers through a drying process. Theobtained fibers were served to stabilization at conditions of 260° C.and 15 minutes. Although the stabilized fiber was tried to be carriedout with carbonization at a carbonization temperature of 1,300° C., thefiber was burned and broken immediately after being introduced into afurnace, and could not be carbonized as a carbon fiber.

Comparative Example 3

Polymer solution for spinning (b) was wet spun in a manner similar tothat in Example 1 to obtain fibers through a drying process. Theobtained fibers were served to stabilization at conditions of 240° C.and 15 minutes. Although the stabilized fiber was tried to be carriedout with carbonization at a carbonization temperature of 1,300° C., thefiber was burned and broken immediately after being introduced into afurnace, and could not be carbonized as a carbon fiber.

Comparative Example 4

Polymer solution for spinning (b) was wet spun in a manner similar tothat in Example 1 to obtain fibers through a drying process. Theobtained fibers were served to stabilization. The fiber was stabilizedat conditions of 280° C. and 15 minutes. Although a fusion happened atthe stage of the stabilization, the fiber was carbonized as it was.Although the stabilized fibers were tried to be carried out withcarbonization at a carbonization temperature of 1,300° C., most of thefibers were burned and broken in a furnace. As the result of tensing asingle fiber with respect to parts barely taken as carbon fibers, thetensile strength was reduced to 1.3 GPa, the degree of elongation was1.0%, and they were very low tensile strength and degree of elongationto cause poor results.

Comparative Example 5

Polymer solution for spinning (b) was wet spun in a manner similar tothat in Example 1 to obtain fibers through a drying process. Althoughthe obtained fibers were tried to be carried out with stabilization atconditions of 300° C. and 15 minutes, they were burned and broken in afurnace for stabilization.

Comparative Example 6

Polymer solution for spinning (b) was wet spun in a manner similar tothat in Example 1 to obtain fibers through a drying process. Althoughthe obtained fibers were tried to be carried out with stabilization atconditions of 360° C. and 15 minutes, they were burned and broken in afurnace for stabilization.

Comparative Example 7

Polymer solution for spinning (c) was treated in a manner similar tothat in Example 1 to obtain fibers. The obtained fibers were served toburning at conditions similar to those in Example 7 to obtain carbonfibers. Because a nitro compound was left in the polymer solution forspinning, as the result of TEM observation, the obtained carbon fiberhad a 2-layer sheath-core structure. With respect to Lc, it was 1.7 nmat the sheath and 1.5 nm at the core. With respect to orientation degreef, the sheath was oriented at 0.86, and the core was oriented at 0.83 orless. As the result of tensing a single fiber, the tensile strength was1.9 GPa, and the degree of elongation was 0.8%. In particular, thedegree of elongation was greatly reduced as compared with Example 8, andit was a poor result.

Comparative Example 8

Polymer solution for spinning (a) was wet spun at a number of filamentsof 12,000 to obtain fibers through a drying process, in a manner similarto that in Example 1. The obtained fibers were served to stabilizationat conditions of 300° C. and 5 minutes similar to those in Example 1,using a hot air circulation drier equipped with no infrared heater, andcarbonization was carried out at a carbonization temperature of 1,300°C.

As the result of TEM observation, the obtained carbon fiber hadsubstantially a 2-layer sheath-core structure. With respect to Lc, itwas 1.6 nm at the sheath and 2.2 nm at the core. With respect toorientation degree f, the sheath was oriented at 0.80, and the core wasoriented at 0.6 or less. As the result of tensing a single fiber, thetensile strength was 1.8 GPa, and the degree of elongation was 1.0% andmuch lower than that in Example 1, and occurrences of fluff was alsohigh.

Comparative Example 9

Polymer solution for spinning (a) was wet spun at a number of filamentsof 12,000 to obtain fibers through a drying process, in a manner similarto that in Example 1. The obtained fibers were served to stabilizationat conditions of 300° C. and 5 minutes similar to those in Example 1,using only an infrared heater (without hot air circulation), andcarbonization was carried out at a carbonization temperature of 1,300°C., but yarn breakage happened because of unevenness of treatment.

The polymer solutions for spinning (a) to (e) used in theabove-described respective Examples and Comparative Examples are shownin Table 1, the conditions and results of Examples 1 to 11 are shown inTable 2, and the conditions and results of Comparative Examples 1 to 9are shown in Table 3, respectively.

TABLE 1 Polymer solution for spinning a b c d e Raw materialacrylonitrile homopolymer part by 11 15 11 10 11 nitrobenzene weight 2 21.5 1.5 monoethanol amine 5 2 3 7 Polar solvent dimethyl sulfoxide 82 8585 85.5 80.5 Conditions dissolution or reaction ° C. 150 80 150 151 152for reaction temperature dissolution or reaction time h 6 6 6 10 7Properties of residual rate of nitro compound % 0 0 24 0 0 polymerMark-Houwink a 0.21 0.5 0.22 0.4 0.07 solution

TABLE 2 Exam Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ExampleExample Unit ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 ple 7 ple 8 ple 9 10 11Kind of polymer PAN(a) PAN(a) PAN(a) PAN(a) PAN(a) PAN(a) PAN(a) PAN(a)PAN(d) PAN(e) PAN(a) solution for spinning Conditions Temperature for °C. 300 320 340 360 300 360 300 360 360 360 360 for burning stabilizationTime for min 5 5 5 5 10 10 15 15 15 15 30 stabilization Time for ° C.1300 1300 1300 1300 1300 1300 1300 1300 1300 1300 1300 carbonization TEMStructure 3-layer 3-layer 3-layer 3-layer 3-layer 3-layer 3-layer3-layer 3-layer 3-layer 3-layer analysis sheath/ sheath/ sheath/ sheath/sheath/ sheath/ sheath/ sheath/ sheath/ sheath/ sheath/ core core corecore core core core core core core core Crystal Sheath nm 1.6 1.6 1.61.6 1.6 1.6 1.6 1.6 1.4 1.5 1.5 size Lc Intermediate layer 1.8 1.8 1.81.8 1.8 1.8 1.8 1.8 1.6 1.8 1.8 Core 2.1 2.1 2.1 2.2 2.1 2.1 2.0 2.1 1.82.1 2.1 Orientation Sheath 0.86 0.86 0.86 0.86 0.86 0.86 0.86 0.85 0.820.80 0.80 degree f Intermediate layer 0.89 0.89 0.89 0.89 0.89 0.89 0.890.88 0.84 0.82 0.82 Core 0.56 0.55 0.55 0.55 0.55 0.55 0.56 0.54 0.540.54 0.54 Rate of flat yarn 70% 80% 70% 80% 80% 80% 70% 70% 60% 90% 70%Tensile Strength GPa 2.1 2.1 2.2 2.2 2.2 2.4 2.3 2.4 2.0 1.6 1.7strength of Degree of % 1.7 1.6 1.5 1.5 1.6 1.6 1.6 1.6 1.3 1.6 1.5single fiber elongation

TABLE 3 Com- Com- Com- Com- Com- Com- Com- Com- Com- parative parativeparative parative parative parative parative parative parative UnitExample 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7Example 8 Example 9 Raw Kind of PAN(a) PAN(a) PAN(b) PAN(b) PAN(b)PAN(b) PAN(c) PAN(a) PAN(a) material polymer solution for spinningConditions Apparatus A A A A A A A B C for burning Temperature ° C. 240260 240 280 300 360 300 300 300 for stabilization Time for min 15 15 1515 15 15 15 5 5 stabilization Time for ° C. 1300 1300 1300 1300 13001300 1300 1300 1300 carbonization TEM Structure F2 F2 F2 hollow F1 F12-layer 2-layer F2 analysis sheath/core sheath/core Crystal Sheath nm —— — 1.8 — — 1.7 1.6 — size Lc Intermediate — — — none — — none none —layer Core — — — none — — 1.5 2.2 — Orientation Sheath — — — 0.86 — —0.86 0.86 — degree f Intermediate — — — 0.89 — — none none — layer Core— — — — — — 0.83 0.56 — Rate of flat yarn — — — 0% — — 40% 30% — TensileStrength GPa — — — 1.3 — — 1.9 1.8 — strength of Degree of % — — — 1.0 —— 0.8 1 — single fiber elongation Apparatus for burning conditions: A;hot air circulation drier equipped with infrared heater, B; hot aircirculation drier, C; infrared heater F1: fused or cut by being molten,impossible in stabilization as fiber bundle, F2: burnt in furnace,impossible in carbonization

The PAN-based carbon fiber and the production method therefor can beapplied to production of any PAN-based carbon fiber required withshortening of time for stabilization and a high degree of elongation.

1.-12. (canceled)
 13. A polyacrylonitrile (PAN)-based carbon fibercomprising three or more phases different in crystal size.
 14. ThePAN-based carbon fiber according to claim 13, wherein respective phasesare layered.
 15. The PAN-based carbon fiber according to claim 14,wherein said carbon fiber has a sheath-core structure having three ormore layers, and satisfies conditions A to D: A: in a sectional area ina direction perpendicular to a fiber axis, an area occupied by a coreoccupies 10 to 70% of the whole of said sectional area, B: a thicknessof a sheath is 100 nm to 10,000 nm, C: a thickness of an intermediatelayer is more than 0 and 5,000 nm or less, and D: a diameter in saiddirection perpendicular to said fiber axis is 2 μm or more.
 16. ThePAN-based carbon fiber according to claim 14, wherein said carbon fiberhas a sheath-core structure having three or more layers, and satisfiesconditions E to H: wherein a crystal size of a core is Lc1, a crystalsize of a sheath is Lc2, and a crystal size of an intermediate layer isLc3. E: Lc1/Lc3≧1.05, F: Lc1/Lc2≧1.05, G: 1.0≦Lc1≦7.0 nm, and H:Lc2≠Lc3.
 17. The PAN-based carbon fiber according to claim 15, whereinan orientation degree f of a crystal of a core is 0.7 or less.
 18. ThePAN-based carbon fiber according to claim 15, wherein said carbon fiberhas a sheath-core structure having three or more layers, and satisfiesconditions E to H: wherein a crystal size of a core is Lc1, a crystalsize of a sheath is Lc2, and a crystal size of an intermediate layer isLc3. E: Lc1/Lc3≧1.05, F: Lc1/Lc2≧1.05, G: 1.0≦Lc1≦7.0 nm, and H:Lc2≠Lc3.
 19. The PAN-based carbon fiber according to claim 16, whereinan orientation degree f of a crystal of a core is 0.7 or less.
 20. Amethod of producing a PAN-based carbon fiber according to claim 13comprising: spinning a solution of a polymer prepared by modifying PANwith an amine-base compound and oxidizing it with a nitro compound toprepare a spun fiber; performing stabilization of said spun fiber in airat 280° C. or higher and 400° C. or lower for 10 seconds or more and 15minutes or less; and thereafter, performing carbonization.